In the particle beam therapy, therapy is implemented by irradiating a charged particle beam (particle beam) onto a diseased site, which is a therapy subject, so as to kill the tissues of the diseased site; in order to deliver a sufficient dose to the tissues of the diseased site without causing damage to the peripheral tissues thereof, there is required a particle beam therapy system that can appropriately control an irradiation dose and irradiation volume. In a particle beam therapy system utilizing a scanning irradiation method among methods of forming the irradiation volume, by scanning a charged particle beam supplied from an accelerator while positioning it by means of a scanning electromagnet and the like, there is performed irradiation of a predetermined dose for each small irradiation area (spot) so that an irradiation field is formed stepwise. In the case of, among scanning irradiation methods, the raster-scanning irradiation method and an irradiation method in which the raster-scanning irradiation method and the spot-scanning irradiation method are combined so that beam cutoff between spots is eliminated, a charged particle beam is irradiated without being cut off until irradiation onto a single slice is completed.
In contrast, in the case where the irradiation subject is an organ that moves with respiration, it is required to implement or interrupt the irradiation of a charged particle beam in synchronization with the respiratory phase so that irradiation can be performed only in a respiration state where the organ moves little and the position of the organ can readily be located. Also in the case where beam cutoff operation is performed at a completion time, of irradiation onto a last spot, that is a last time point for scanning irradiation onto each slice, it is required to cut off a beam. However, it is difficult to instantaneously cut off a charge particle beam emitted from an accelerator; therefore, a very small quantity of dose, which is referred to as a leakage dose, is irradiated in a period (although short) after the cutoff operation is started. Accordingly, a leakage dose is unnecessarily delivered to the spot where the cutoff operation is performed; therefore, there has been a problem that it is difficult to implement appropriate dose delivery conforming to a treatment plan. Thus, there has been proposed a particle beam therapy system (e.g., refer to Patent Document 1) in which in the transport path that connects an accelerator and an irradiation apparatus, there are provided a deflection electromagnet for changing a beam orbit and a beam dump for dumping a charged particle beam whose orbit has been changed so that the time required for cutting off a beam is shortened and hence a leakage dose is suppressed.